Sample loading into biological instruments and devices with fluidic systems currently requires large sample volumes, typically greater than 100 μL. In biology and science, samples are precious and often there is a desire to use as small a volume as possible. This is exemplified in the “world-to-chip” problem, where microfluidic devices are capable of handling volumes <1 μL, but it is difficult to load a sample of that size into the device. The “world” can include biological, chemical, and clinical samples that are derived from experiments, blood, and other biological fluidics. The ability to fabricate increasingly smaller channels in glass, fused silica, and other materials through photolithography further emphasizes the importance of solving this problem.
This sample loading problem is present in flow-based systems, such as flow cytometers, where a moving fluid flow carries a sample to a laser detection region for analysis. Current methods of sample loading for a flow cytometer, or similar types of flow-based instruments, include utilizing a sample withdrawal tube for “sipping” the sample. This approach typically requires 200 μL of sample, of which only 50 μL may be withdrawn and analyzed. While the sample injection tube is flushed between uses, there is still the possibility of cross-contamination. This leads to a waste of 150 μL of sample, which for fear of contamination cannot be returned to its original source.
Lab-on-valve devices also require sample injection. In these systems, a rotating valve allows selection among the different types of samples introduced into the system. Sample introduction is typically accomplished using syringes attached to the inlet ports. Microvolume syringes exist but they are typically expensive for single-use applications.
Sample injection is also important in various types of chromatography. In chromatography systems, after the sample is injected by the syringe, there is sometimes a rotary diaphragm valve that is utilized to load precise volumes of samples into flow based systems. A certain sample volume is introduced by the syringe. This sample fills a section of channel. The diaphragm valve is rotated to bring the filled channel to connect with a carrier fluid and the entrance to the column. Movement of the carrier fluid displaces the sample. This approach allows a precise amount of volume to be delivered to the system. The diaphragm valve approach for sample loading thus requires a syringe for sample injection and a rotary valve for fluid metering,
The presence or absence of bubbles at either end of the sample can impact the loading profile of the sample into the system. The desired loading profile for a sample can take one of several configurations: bubble in front, bubble in back, bubble on both sides, and no bubbles. The bubble interface introduces plug flow on that side of the sample, whereas no bubble leads to a stretched sample as it goes through the system. No bubble on both sides leads to a long transit time for the sample through the system and the greatest amount of sample dilution. A long transit time may not be desirable if the sample analysis time is to be short. Conversely, bubbles on both sides may not be desirable if some sample dilution is required. The ideal sample loading may be a bubble on one end, preferably the back end of the sample, such that the front end has a parabolic profile and some sample dilution.
A diversity of methods can be utilized to address this problem, including inserting a capillary in-line with tubing, using U-shaped consumables, dummy consumables, in-line loaders, and variations thereof. One can also exclude a bubble based on inserting a pin into the capillary as well as making one end of the capillary contact first prior to the other end. These approaches are complex and further simplicity is desirable.